Field of the Disclosure
The present application relates to a liquid crystal lens film structure, a method of fabricating the same and an image display device including the same. More particularly, the present application relates to a liquid crystal lens film structure with an enhanced adhesive property and an enhanced liquid crystal alignment property, a method of fabricating the same and an image display device including the same.
Description of the Related Art
Stereoscopic three-dimensional (3-D) image display devices each provide different images to the right and left eyes of a user and enable the user to feel a sense of distance and a three-dimensional visual effect from a displayed image. Such stereoscopic 3-D image display devices are classified into glasses and glasses-free types according to whether or not a user wears glasses.
The glasses-type stereoscopic 3-D image display device causes users viewing a 3-D image to each wear glasses. Due to this, the glasses-free stereoscopic 3-D image display device is being recently preferred rather than the glasses-type stereoscopic 3-D image display device.
The glasses-free stereoscopic 3-D image display device can be broadly distinguished into a mode using a parallax barrier and another mode using a liquid crystal lens array. The glasses-type stereoscopic 3-D image display device using the parallax barrier had been actively researched. However, when a 3-D image is displayed, the glasses-type stereoscopic 3-D image display device using the parallax barrier deteriorates the definition of a displayed image by half and increases viewing points of the displayed image. As such, the glasses-type stereoscopic 3-D image display device using the parallax barrier largely deteriorates its definition and brightness. In view of this point, a need exists to develop the glasses-free stereoscopic 3-D image display device which employs the liquid crystal lens array.
The liquid crystal lens array of the related art used in the glasses-free stereoscopic 3-D image display device mainly employs glass substrates as upper and lower substrates. However, the glass substrate must have a large weight and thickness. Due to this, the liquid crystal lens array cannot become lighter and thinner. Also, the liquid crystal lens array increases the fabrication cost unlike that using a low-cost film material.
To address this matter, a liquid crystal lens array using flexible substrates instead of the glass substrates is developed. Such a structure of the liquid crystal lens array used in a glasses-free stereoscopic 3-D image display device according to the related art is shown in FIG. 1.
As shown in FIG. 1, the liquid crystal lens array includes a first flexible substrate and a second flexible substrate disposed to face each other. First and second transparent electrode layers are formed on opposite (or inner) surfaces of the first and second flexible substrates. Also, the liquid crystal lens array includes a liquid crystal lens film interposed between the first flexible substrate and the second flexible substrate. Moreover, the liquid crystal lens array includes an adhesive layer interposed between the second flexible substrate and the liquid crystal lens film, and a PI (polyimide) alignment film configured to align liquid crystal molecules of the liquid crystal lens film.
The liquid crystal lens film includes a lens portion with a lenticular lens shape and a non-lens portion. The liquid crystal molecules are aligned in a fixed direction by the PI alignment film, and realigned in another fixed direction when a voltage is applied between the first and second transparent electrode layers. The refractive index of the lens portion is varied along the change of the alignment direction of the liquid crystal molecules. As such, a progressive direction of light projected from the display device is changed by the lens portion.
FIG. 2 is a cross-sectional view schematically illustrating a method of displaying two-dimensional (2-D) and 3-D images by the glasses-free stereoscopic 3-D image display device which uses the related art liquid crystal lens array. Referring to FIG. 2, when any electric field is not applied to the liquid crystal lens film, the lens portion has the same refractive index as an ordinary refractive index of the liquid crystal and the non-lens portion has the same refractive index as an extra-ordinary refractive index of the liquid crystal. In other words, the lens portion and the non-lens portion have the different refractive indexes. As such, light passing through the liquid crystal lens film is refracted in an interface between the lens portion and the non-lens portion and generates a parallax. In accordance therewith, a 3-D image can be realized. On the other hand, when an electric field is applied to the liquid crystal lens film, the lens portion has the same refractive index as the ordinary refractive index of the liquid crystal. Then, any refractive index difference is not generated between the lens portion and the non-lens portion. As such, light passes through the liquid crystal lens film without any refraction. In accordance therewith, the 2-D image can be realized (or displayed). In this manner, the glasses-free stereoscopic 3-D image display device can selectively display the 2-D image and the 3-D image using the liquid crystal lens array.
As currently known, an adhesive strength between films of the liquid crystal lens array is weak and several defects must be generated. Actually, the films of the liquid crystal lens array are easily to peel off, and a liquid crystal material is easy to spread by a pressure which is applied at a combination of a display panel and the liquid crystal lens array.
Also, when the liquid crystal lens array includes flexible substrates such as film substrates, it is difficult to perform a high temperature process due to a low transition temperature Tg of the film substrate. Actually, a curl defect is generated in the film substrate at a high temperature polyimide process which is performed for aligning liquid crystal molecules. Due to this, the liquid crystal cannot have a desired refractive index when an electric field is applied. FIGS. 3, 4A, 4B, 5A and 5B are photographs illustrating problems generated in the structure of the related art liquid crystal lens array which includes the flexible substrates.
Moreover, an adhesive material of the adhesive layer within the related art liquid crystal lens array reduces an aligning force which is used to align the liquid crystal molecules. Due to this, an image quality property deteriorates. Furthermore, because the adhesive layer with a non-conductive property is formed on the second flexible substrate, a driving voltage necessary to apply an electric field to the liquid crystal material becomes higher. Therefore, some solutions to address these problems associated with the related art are desired.